Light tells the story of the universe. Since light takes time to travel, the farther out into space we look:

the further back in time we see

the further into the future we see

we see things that happened just recently

Nope. Try again!

You got it!

ANSWER: the further back in time we see

When we flip a switch we see the light almost instantly. But sunlight is 8 minutes old, and light from galaxies can be millions or even billions of years old! Light that is 13.8 billion light years away (13.8 billion ly old) has reached Earth from every direction. Astronomers call this sphere the observable universe.

Question 1 of 10

NEXT QUESTION

2

What is the most distant thing in space that we can see without a telescope?

the Orion Nebula, a stellar nursery in our Milky Way

the Andromeda galaxy, our closest neighbor

the Virgo cluster, a collection of about 2000 galaxies

Nope. Try again!

You got it!

ANSWER: the Andromeda galaxy, our closest neighbor

Andromeda is over 2.5 million light-years away. But compared to the size of the universe, that distance is tiny.

Question 2 of 10

NEXT QUESTION

3

In the 1920s, Edwin Hubble and fellow astronomers at Mount Wilson Observatory were the first to discover galaxies beyond the Milky Way. What else did they discover?

distant galaxies move farther and farther away from us

distant galaxies look like the Milky Way

distant galaxies give off light

Nope. Try again!

You got it!

ANSWER: distant galaxies move farther and farther away from us

It seemed the universe was stretching or expanding. This was the first evidence for the Big Bang theory.

Question 3 of 10

NEXT QUESTION

4

According to the Big Bang theory, the universe was born about 13.8 billion years ago. At the moment of the Big Bang, the universe was:

hot and dense like the center of a star

hot and dense like the center of Earth

scientists have no idea

Nope. Try again!

You got it!

ANSWER: hot and dense like the center of a star

The very early universe was very hot, very dense, and very bright, like the center of a star. Everywhere was starlight!

Question 4 of 10

NEXT QUESTION

5

In 1964, astronomers working with a radio telescope in New Jersey accidentally discovered something that came to be known as the cosmic microwave background. What was it?

a bright white light from the Big Bang

low energy radiowaves from the Big Bang

a loud noise from the Big Bang explosion

Nope. Try again!

You got it!

ANSWER: low energy radiowaves from the Big Bang

The astronomers thought the faint signal might have been pigeon poop on their telescope. But they actually detected the afterglow of light from the early universe — the oldest light ever seen. Only a radio telescope could pick up the longer wavelengths of the cosmic microwave background.

Question 5 of 10

NEXT QUESTION

6

In 2009, the Planck satellite was launched into space to measure light in the microwave band. It orbits:

the Moon

Earth

the Sun

Nope. Try again!

You got it!

ANSWER: the Sun

Planck orbits the Sun about 1.5 million km (1 million mi) from Earth, at a point where Earth and the Sun's gravity balance each other exactly. Their combined gravity means that anything near this location orbits the Sun in exactly one year, and stays in the same place relative to Earth.

The 'c' stands for the speed of light, or 670 million miles per hour. Since 670 million squared is a huge number, this means that an object with just a little mass holds a huge amount of energy! The letter 'c' comes from the Latin word celeritas, which means 'swiftness.'

Question 7 of 10

NEXT QUESTION

8

Normal matter is everything we can touch and see, including dogs, trees, planets, stars, and galaxies. It makes up how much of the total mass-energy in our universe?

less than 5 percent

about 25 percent

more than 70 percent

Nope. Try again!

You got it!

ANSWER: less than 5 percent

More than 95% of the universe is made up of stuff we can't see; Dark matter, an invisible substance, makes up about 25%. A mysterious pressure called dark energy makes up about 70%.